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Isotopes of hydrogen. Illustration showing the isotopes of hydrogen. Isotopes are forms of an element that contain different numbers of neutrons in the atomic nucleus. Hydrogen-1, known as protium, has one proton (red) and no neutrons in its nucleus, hydrogen-2, deuterium, has one proton and one neutron (blue), and hydrogen-3, tritium, has one proton and two neutrons . All the isotopes also have one electron (black) orbiting the nucleus. Tritium is a radioactive isotope, but protium and deuterium are stable.